1. Field of the Invention
The present invention is concerned with novel cyclohexanecarbonitriles, their manufacture, liquid crystalline mixtures which contain these compounds as well as their use for electro-optical purposes.
2. Description
Liquid crystals have recently gained considerable importance as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects such as, for example, the dynamic scattering, the deformation of aligned phases (DAP type) the Schadt-Helfrich effect (twisted-nematic cell), the guest/host effect (guest/host cell) or a cholesteric-nematic phase transition (phase change effect).
The liquid crystals which are used must have a good stability towards heat, moisture, air, electromagnetic radiation, electrical fields and the like. Further, they should be colourless, should have short response times and low viscosity, should give a good contrast and should have a nematic or cholesteric mesophase in the entire temperature range in which the liquid crystal cell is to be operated. Since liquid crystals are usually used as mixtures, it is, moreover, important that the components have a good miscibility with one another and at the same time form a nematic or cholesteric mesophase. Other properties such as, for example, the electrical conductivity, the threshold potential, the multiplexibility and the dielectric anisotropy must fulfill different conditions depending on the type of cell which is used.
Liquid crystals with negative anisotropy of the dielectric constants (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. less than 0, .epsilon..sub..perp. signifying the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto) can be used, for example, in liquid crystal cells of the light-scattering type, in DAP cells or in guest/host cells. Further, a two-frequency matrix addressing has been proposed (e.g. German Offenlegungsschriften Nos. 2856134 and 2907940, corresponding respectively to U.S. Pat. No. 4,417,785 and U.K. Pat. No. 2,020,075) in order to improve the multiplex ratio in the multiplex control of liquid crystal devices, especially of twisted-nematic cells and guest/host cells. In order to improve the dielectric anisotropy above the "cross-over frequency" (dielectric relaxation frequency at which .epsilon..sub..parallel. =.epsilon..sub..perp.), components with negative dielectric anisotropy can be added to the corresponding dielectrics (e.g. German Offenlegungsschrift 3221462, corresponding to U.S. Pat. No. 4,460,770).
Compounds with a large negative anisotropy of the dielectric constants are already known, examples of such compounds being pyridazines, pyridazine oxides and compounds containing a 2,3-dicyano-1,4-phenylene group. However, these compounds have various disadvantages such as e.g. poor solubility in mixtures, high viscosities, high melting points, strong smectic tendencies, chemical instability, high electrical conductivity or large clearing point depressions in mixtures and are therefore usable only to a very limited extent.